These two paths, the bright and the dark, are held to be eternal for the world. By one a person goes and does not return; by the other, a person comes back again.
Explanation
Krishna is closing his teaching on the two paths a departing soul may take: the bright path (śukla), described in the preceding verses as the way of light, and the dark path (kṛṣṇa), the way of smoke and night. He says both are ancient and ever-present in the world's order. One leads to freedom from rebirth (anāvṛtti), the other to return. Traditions differ on whether these paths describe literal times of death or stages and kinds of spiritual attainment.